Converters which convert the alternating current (AC) from the mains (e.g., single-phase 230 volts in Europe) to a direct current (DC) are used in a great variety of applications, for example, such as controlling DC motors for household or industrial use (e.g., in washing machines, refrigerators, dishwashers, industrial machines). Such converters are also known as "Switch Mode Power Supply" (SMPS). Converters can comprise a rectifier bridge and a switch-coil arrangement to periodically boost energy to an output capacitor. Construction and operation are explained, for example, in the following reference: "U. Tietze and Ch. Schenk: Halbleiter-Schaltungstechnik', Zehnte Auflage (10th edition), Springer Verlag, Berlin, Heidelberg, New York, ISBN 3-540-56184-6, sections 18.5 to 18.7 on pages 561-586". The 9th edition of this book was published in English in 1991 under the title "Electronic Circuits, Design and Applications".
Load conditions, such as the amount of electrical energy required by the motor, can vary in a short time, for example, when the motor instantaneously has to increase its mechanical torque. To ensure motor operation, the converter has to supply energy to the motor always within the borders of predetermined minimum and maximum values. Otherwise, converter and/or motor could create a safety hazard. For example, the motor could reverse its rotating sense; or the motor could turn into a generator causing unwanted current backflow into the converter.
Hence, the capacitor must be large enough to forward energy to the motor even if the switch-coil arrangement is--due to feedback time constraints--tenporarily not able to boost the capacitor. The size of the capacitor is, however, an important cost factor and should desirable be kept as small as possible.
There are other requirements to a converter. Since the coil drains current from the mains without following the sine function, distortions can go back to the mains. This is not wanted. If, for example, the converter input is derived from conductor pair in a three-phase supply system (as it is widespread in Europe with three power conductor and a neutral conductor), current imbalance between the conductors can lead to an unwanted current in the neutral conductor. As a consequence, a circuit breaker could open, or iron losses in power transformers at the mains could be increases. Especially the third harmonic (e.g., 150 Hz) or higher harmonics plays an important role. Distortions caused by the converter have to be within the limits given by government regulations, national laws, and international standards.
While for AC motors it is easily possible to compensate power factors (ratio between real power and total apparent power, cos .phi. by additional capacitors switched to the mains; such an approach is not suitable for DC motors.
It will in explained in the following why converters of the prior art suffer from high distortions, large capacitors, or low power factors. The present invention seeks to provide a converter and a method to operate it which mitigate or disadvantages and limitations of the prior art.